AKA point, period, dot [. ]
The beginning of a sentence is marked in English by a capitalised first letter, and between the capital letter and the full stop we have a written space into which we insert words to convey ideas.
The mark signals the end of a sentence, meaning that all the ideas which the writer wishes to communicate in that group of words have been communicated. Better still, just one idea will be conveyed memorably.
British English tends to say “full stop” while US English has “period”. The older word, period, has the idea (from Gk) of “from beginning to end”, as in having come full circle and presumably of getting ready to start again. We are a verbose species, after all. It’s good that we have something to make us stop.